Magnetic particle inspection
The magnetic particle testing (also magnetic powder crack test, fluxing Fluxprüfung or called) is a method for the detection of cracks in or near the surface of ferromagnetic materials .
The workpiece must be magnetized for the test . In the case of large workpieces for which complete magnetization is not possible, only the partial area to be tested is magnetized. The field lines created by the magnetization run parallel to the surface. Cracks and imperfections close to the surface that are perpendicular to the field lines generate a magnetic stray field. This means that the field lines emerge from the ferromagnetic material on one side of the defect and re-enter on the other side. This results in the creation of magnetic poles . If iron powder is now distributed over this stray field, it will accumulate at the defect because it is attracted by the magnetic effect. Cracks that run parallel to the field lines do not generate a stray field and therefore cannot be detected. Pores and cracks below the surface can only be localized to a certain depth.
There are different methods of magnetizing components.
Current flow
With the current flowing through the workpiece to be tested, a current flows through it. This current creates a ring-shaped magnetic field . Longitudinal cracks on the test specimen are thus perpendicular to the magnetic field lines and generate the necessary stray field.
In other words: the current flow shows “longitudinal cracks”
Field flooding
In contrast to the current flooding in the generated field flow a magnetic flux in the test object without current flows in it.
With the help of one or more current-carrying coils, a magnetic field is generated in a U-shaped iron yoke. The workpiece is clamped in this iron yoke. This creates a magnetic field in the longitudinal direction of the component. Cracks lying at right angles to it, "transverse cracks", form a leakage flux and are displayed.
Combined process
With many test objects, not only cracks in a certain preferred direction can be expected. Then either several tests, i.e. several observations in a row, or combined crack detection methods must be used.
The simplest combination in terms of equipment consists of direct current yoke magnetization and alternating current flow. The test devices are designed so that the current is fed into the workpiece via the poles of the magnetic yoke at the same time. The magnetic yoke must be electrically interrupted once in order to prevent a shunt.
Optical indication of the crack
In the magnetic particle crack test, the iron particles are applied during magnetization. Fine powdery particles, often ferromagnetic iron oxides , reveal even the finest hairline cracks. The powder particles are suspended in suitable liquids, such as, for example, water, and poured or sprayed over the test object during the magnetization.
The magnetic particle crack test can be carried out in daylight or in the dark with fluorescent test equipment. In daylight, black or very strongly fluorescent test equipment is usually used in order to obtain a high-contrast crack indication. For fluorescent testing in the dark (<20 lux ambient brightness), only fluorescent testing equipment is used. Fluorescent magnetic powder particles are associated with color pigments that glow bright yellow, green or red when irradiated with light in the wavelength range between approx. 300 - 500 nm. The resulting improvement in contrast enables the crack to be recognized much better.
Viewing condition
In industry, the viewing conditions for test specimens are specified in a standard (EN ISO 3059).
Sources of excitation for fluorescent testing
According to EN ISO 3059, a standard-compliant, fluorescent test must use a UV-A radiation source or a blue light to stimulate the fluorescence. While discharge lamps (mercury vapor, xenon or metal halide lamps) have been used exclusively in the last few decades, UV-A LED lamps or blue light lamps are primarily used today.
In addition to fluorescence excitation using UV light, blue light (450 nm) can also be used. The use of blue light systems is regulated in DIN CEN / TR 16638 from May 2014.
UV radiation and blue light can seriously endanger the eyes and skin, so a corresponding hazard analysis and adequate protective measures are very important.
UV radiation is a known risk to the eyes and skin. However, the usual radiation exposure when using high-quality UV-A radiation sources and light personal protective equipment (covering clothing and UV protective goggles) is so low within an eight-hour shift that there is no long-term damage and the permissible radiation dose is by far not exceeded. The hazard from pure UV-A radiation, as used for fluorescent surface crack testing, is in itself quite harmless. A greatly increased UV radiation dose due to a lack of protective measures, e.g. B. in the solarium (without protection), where a lot of UV-B and UV-C radiation is emitted, inflammation of the cornea and with an increased unprotected dose can cause cataracts (cataracts) for months, years or even decades. , as well as causing sunburn or skin cancer on the skin.
According to the new OSTrV regulation, UV exposure must be assessed for each employee and documented for 30 years.
Blue light lies in the visible range of the light spectrum and passes through the lens of the eye unhindered and hits the retina . For this reason, a filter that suppresses the high-energy (actinic) blue light (420 - 480 nm) is used in blue light systems, as they have been used for years in fluorescent crack testing. A photochemical hazard (blue light hazard) for the eye is thus excluded. Very strong, high-energy blue light (we know it from the field of welding) can damage or even burn the retina WITHOUT protection ( photoretinitis ), which within a very short time leads to incurable total or partial blindness (minute range) or impairment of color vision (seconds range) can lead. The use of blue light systems is not expected to pose a risk to the skin.
Both types of radiation have a potential hazard, which must be determined and assessed in each individual case. The German Society for Non -Destructive Testing (DGZfP) has issued a leaflet (EM 6) for use in standard workplaces, which enables simple and safe classification under occupational health and safety law. This guideline is available from DGZfP in Berlin or the Beuth Verlag des DIN .
For the assessment of the risk at the workplace, a. the directive 2006/25 / EG regulation optical radiation relevant. This guideline was published on July 27, 2010 in the OSTrV.
Magnetic particle testing is one of the non-destructive testing methods .
Magnetic Particle Testing Standards
- DIN 25435-2, Recurring tests of the components of the primary circuit of light water reactors - Part 2: Magnetic particle and penetrant test
- DIN EN 1330-7, Non-destructive testing - Terminology - Part 7: Terms used in magnetic particle testing
- DIN EN 1369, foundry - magnetic particle testing
- DIN EN 10228-1, Non-destructive testing of steel forgings - Part 1: Magnetic particle testing
- DIN EN 10246-12, Non-destructive testing of steel tubes - Part 12: Magnetic particle testing of seamless and welded ferromagnetic steel tubes for the detection of surface defects
- DIN EN 10246-18, Non-destructive testing of steel pipes - Part 18: Magnetic particle testing of the pipe ends of seamless and welded ferromagnetic steel pipes for the detection of doublings
- DIN EN ISO 3059, non- destructive testing - penetrant testing and magnetic particle testing - viewing conditions
- DIN EN ISO 9934-1, Non-destructive testing - Magnetic particle testing - Part 1: General principles
- DIN EN ISO 9934-2, Non-destructive testing - Magnetic particle testing - Part 2: Test equipment
- DIN EN ISO 9934-3, Non-destructive testing - Magnetic particle testing - Part 3: Devices
- DIN EN ISO 17638, Non-destructive testing of welded joints - Magnetic particle testing
- DIN EN ISO 23278, Non-destructive testing of welded joints - Magnetic particle testing of welded joints - Permissible limits
- DIN CEN / TR 16638, Non-Destructive Testing - Penetrant and magnetic particle testing using blue light